In satellite communication systems, electromagnetic energy signals are beamed from a satellite to the Earth. The beam of signals may cover either a large section of the Earth surface, such as a continent or a country, or a relatively small region. The first technique is generally referred to as area beam coverage and the latter technique is generally referred to as spot beam coverage. Moreover, simultaneous coverage by a plurality of spot beams may also be used. Such a technique is generally referred to as multiple beam coverage. The generation and positioning of such multiple beams is the subject of the present invention.
In general, multiple beam antenna systems are common in the prior art. For example, such systems are disclosed in U.S. Pat. Nos. 3,255,450, by Butler; 4,231,040, by Walker; and 4,315,262, by Acampora et al. More particularly, the multiple beam systems disclosed in both Butler and Walker, supra, are capable of transmitting simultaneously a multiplicity of individual signals. Acampora et al., supra, is capable of transmitting a plurality of spot beams each of which covers a region on the Earth. The prior art multiple beam systems cannot be changed readily from area beam coverage to spot beam coverage. Another deficiency in prior art multiple beam systems is the lack of convenient means for changing the spot beam coverage of a beam. A fortiori, the coverage of the individual signals cannot be changed independently. Another further deficiency in the prior art multiple beam systems is the absence of variable dual mode beam coverage. "Dual mode" in this regard is defined as two independent collections of signals. The collection of signals are generally referred to as "odd" and "even" modes.